Modern silviculture often requires the planting of large numbers of genetically identical plants that have been selected to have advantageous properties. Production of new plants by sexual reproduction, which yields botanic seeds, is usually not feasible. Asexual propagation, via the culturing of somatic or zygotic embryos, has been shown for some species to yield large numbers of genetically identical embryos, each having the capacity to develop into a normal plant. Plant embryos created by in vitro cultures, however, lack the natural protective and nutritive features of natural botanic seeds. Attempts have been made to provide the protective and nutritive structures found in natural botanic seeds to plant embryos cultured in a laboratory by using manufactured seeds. Manufactured seeds are described, for example, in U.S. Pat. Nos. 5,564,224; 5,687,504; 5,701,699; and 6,119,395.
Problems with manufactured seeds remain. Both the rate of successful germination and the quality of germinants produced from manufactured seeds are lower than the rate and quality obtained from natural botanic seeds. One of the problems encountered with manufactured seeds is that excess water may accumulate in the cavity of the manufactured seed that holds a plant embryo, and such condition may result in inhibition of germination, abnormal growth, and/or drowning of the embryo.
The flow of water into the cavity holding the embryo in a manufactured seed is a function of the relative water potentials of the components of the manufactured seed. Water potential is the potential energy of water per unit volume relative to pure water. Water potential is measured in Pascal units, with pure water having a potential of 0 Mpa. Water potential represents the flow of water from one area to another. Water will flow from an area of higher potential to an area of lower, or more negative, potential.
A manufactured seed, generally, comprises a seed coat, nutritive media, a shoot restraint, and a cavity within the shoot restraint that holds a plant embryo. In order for an embryo to survive and thrive in a manufactured seed, the embryo must be able to obtain water and nutrients from nutritive media. The relative water potential of the nutritive media, shoot restraint, and embryo must be such that the embryo has a more negative potential than the shoot restraint, which has a more negative potential than the nutritive media, so that the water and dissolved nutrients will flow from the nutritive media, through the shoot restraint, to the cavity, and to the embryo in the cavity. However, the relative differences in the water potentials of the components of a manufactured seed must be balanced so that the embryo receives a sufficient amount of water and nutrients without an excess amount of water flowing into the cavity so as to be detrimental to the embryo.
Therefore, there is a need to improve the rate of germination and the quality of germinants obtained from manufactured seeds by preventing an excess amount of water from flowing into the cavity of a manufactured seed, in which an embryo is disposed, while providing a sufficient amount of water and nutrients to the embryo. The present invention addresses these and other needs.